Cyclonic separators are well known devices for separating dirt and dust from an air flow. Accordingly, such devices have gained popularity in the field of vacuum cleaners, since they can provide an alternative to the traditional dust bags.
It is well known that the overall separation efficiency of such so-called bagless vacuum cleaners can be improved by providing a first stage comprising a low efficiency cyclone for separating coarse dirt and dust from the airflow, and second stage comprising a higher efficiency cyclone mounted downstream of the first stage for separating finer dust particles from the partially cleaned air. U.S. Pat. No. 2,171,248 discloses one such cyclonic vacuum cleaner, in which the second higher efficiency stage is nested inside an outer annular low efficiency cyclone.
In order to further improve the separation efficiency of bagless vacuum cleaners, it has been proposed to mount a plurality of high efficiency cyclones in parallel downstream of the low efficiency cyclone. International Patent Application WO02/067757 discloses one such upright vacuum cleaner, in which the high efficiency cyclones are mounted in parallel in an annular array above the low efficiency cyclone. A disadvantage of this arrangement is that the overall length of the separation stages is too great for the arrangement to be used in more compact cylinder cleaners. A further disadvantage of the arrangement disclosed in WO02/067757 is that the array of high-efficiency cyclones depends into the low-efficiency cyclone structure, thereby dictating the size of the low-efficiency cyclone and limiting its efficiency.
UK Patent Application GB2406065 discloses a solution to the above-mentioned problem, in which the higher efficiency cyclones are mounted in an annular array concentrically around the low efficiency cyclone. In any cyclonic vacuum cleaner, the majority of the dirt and dust is collected by the low efficiency first stage and it is well known to form at least a portion of the side wall of the collection chamber of the first stage from a transparent material, so that the user can determine the fill level of the cleaner. However, a disadvantage of the arrangement of UK Patent Application GB2406065 is that the mounting of the higher efficiency stages around the lower efficiency stage obscures the user's view of the collection chamber of the first stage.